Organic light emitting diode display

ABSTRACT

An organic light emitting diode display according to an exemplary embodiment includes: a substrate; a first buffer layer on the substrate; a first semiconductor layer on the first buffer layer; a first gate insulating layer on the first semiconductor layer; a first gate electrode and a blocking layer on the first gate insulating layer; a second buffer layer on the first gate electrode; a second semiconductor layer on the second buffer layer; a second gate insulating layer on the second semiconductor layer; and a second gate electrode on the second gate insulating layer.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a continuation application of U.S. patent application Ser. No.16/591,966, filed Oct. 3, 2019, which issued as U.S. Pat. No. 11,257,886on Feb. 22, 2022, the disclosure of which is incorporated herein byreference in its entirety. U.S. patent application Ser. No. 16/591,966claims priority to and benefit of Korean Patent Application No.10-2018-0118769 under 35 U.S.C. § 119, filed on Oct. 5, 2018, in theKorean Intellectual Property Office, the entire contents of which areincorporated herein by reference.

BACKGROUND 1. Field

The present disclosure relates to an organic light emitting diodedisplay.

2. Description of the Related Art

An organic light emitting element of an organic light emitting diodedisplay includes two electrodes and an organic light emitting layerpositioned therebetween. Electrons injected from a cathode that is anelectrode and holes injected from an anode that is another electrode arecombined to each other in the organic light emitting layer to formexcitons. Light is emitted while the excitons discharge energy.

As organic light emitting diode (OLED) display technology is developed,organic light emitting diode (OLED) displays are becoming highresolution. Accordingly, there is a growing need for highly integratedorganic light emitting diode displays.

SUMMARY

An organic light emitting diode display according to an exemplaryembodiment includes: a substrate; a first buffer layer on the substrate;a first semiconductor layer on the first buffer layer; a first gateinsulating layer on the first semiconductor layer; a first gateelectrode and a blocking layer on the first gate insulating layer; asecond buffer layer on the first gate electrode; a second semiconductorlayer on the second buffer layer; a second gate insulating layer on thesecond semiconductor layer; and a second gate electrode on the secondgate insulating layer.

The blocking layer may overlap the second semiconductor layer.

The blocking layer may be at the same layer as the first gate electrode.

The organic light emitting diode display according to the presentexemplary embodiment may further include a driving voltage linetransmitting a driving voltage, and the blocking layer may be connectedto the driving voltage line to receive the driving voltage.

The first semiconductor layer may include: a first channel regionoverlapping the first gate electrode; and a first source region and afirst drain region at respective sides of the first channel region, andthe second semiconductor layer may include: a second channel regionoverlapping the second gate electrode; and a second source region and asecond drain region at respective sides of the second channel region.

The blocking layer may overlap the second channel region.

A third gate insulating layer on the second gate electrode, and a secondstorage electrode on the third gate insulating layer and overlapping afirst storage electrode of the second gate electrode, may be furtherincluded.

A first source electrode and a first drain electrode respectivelyconnected to the first source region and the first drain region, and asecond source electrode and a second drain electrode respectivelyconnected to the second source region and the second drain region, maybe further included, and the second storage electrode may be connectedto the second drain electrode.

A pixel electrode on the second storage electrode, an organic emissionlayer on the pixel electrode, and a common electrode on the organicemission layer may be further included, and the second drain electrodemay be connected to the pixel electrode.

An encapsulation layer on the common electrode may be further included.

A first source electrode and a first drain electrode respectivelyconnected to the first source region and the first drain region, and asecond source electrode and a second drain electrode respectivelyconnected to the second source region and the second drain region, maybe further included, and the second storage electrode may be connectedto the driving voltage line.

One of the first semiconductor layer and the second semiconductor layermay be an oxide semiconductor and the other may include polysilicon.

An organic light emitting diode display according to an exemplaryembodiment includes: a substrate; a first transistor on the substrate; asecond transistor on the first transistor; and a blocking layer underthe second transistor, wherein the first transistor includes: a firstsemiconductor layer on the substrate; and a first gate electrode on thefirst semiconductor layer, wherein the blocking layer is on a layeroverlying the first semiconductor layer.

A buffer layer between the first transistor and the second transistormay be further included, and the second transistor may include a secondsemiconductor layer on the buffer layer, and a second gate electrode onthe second semiconductor layer.

The second semiconductor layer may include a channel region overlappingthe second gate electrode, and a source region and a drain region atrespective sides of the channel region, and the blocking layer mayoverlap the channel region.

The organic light emitting diode display according to the presentexemplary embodiment may further include a driving voltage linetransmitting a driving voltage, and the blocking layer is connected tothe driving voltage line to receive the driving voltage.

The blocking layer may be in the same layer as the first gate electrode.

The first transistor may include a third gate electrode on the firstgate electrode and overlapping the first gate electrode, and theblocking layer may be on the same layer as the third gate electrode.

The blocking layer may also overlap the source region and the drainregion of the second semiconductor layer.

One of the first semiconductor layer and the second semiconductor layermay be an oxide semiconductor and the other may include polysilicon.

BRIEF DESCRIPTION OF THE DRAWINGS

Features will become apparent to those of skill in the art by describingin detail exemplary embodiments with reference to the attached drawingsin which:

FIG. 1 illustrates an organic light emitting diode display according toan exemplary embodiment.

FIG. 2 illustrates an equivalent circuit diagram of one pixel of anorganic light emitting diode display according to an exemplaryembodiment.

FIG. 3 illustrates a cross-sectional view of an organic light emittingdiode display according to an exemplary embodiment.

FIG. 4 illustrates a graph showing an instantaneous afterimage durationof an organic light emitting diode display according to a comparativeexample and an exemplary embodiment.

FIG. 5 illustrates an equivalent circuit diagram of one pixel of anorganic light emitting diode display according to another exemplaryembodiment.

FIG. 6 illustrates a cross-sectional view of an organic light emittingdiode display according to another exemplary embodiment.

FIG. 7 illustrates a cross-sectional view of an organic light emittingdiode display according to another exemplary embodiment.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter withreference to the accompanying drawings; however, they may be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey exemplary implementations to those skilled in the art.

In order to clearly explain the embodiments, elements not directlyrelated to thereto are omitted, and the same reference numerals areattached to the same or similar constituent elements through the entirespecification.

In addition, the size and thickness of each configuration shown in thedrawings are arbitrarily shown for better understanding and ease ofdescription. In the drawings, the thickness of layers, films, panels,regions, etc., are exaggerated for clarity. In the drawings, for betterunderstanding and ease of description, the thicknesses of some layersand areas are exaggerated.

It will be understood that when an element such as a layer, film,region, or substrate is referred to as being “on” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” another element, there are no intervening elements present. Further,in the specification, the word “on” or “above” means positioned on orbelow the object portion, and does not necessarily mean positioned onthe upper side of the object portion based on a gravitational direction.

In addition, unless explicitly described to the contrary, the word“comprise” and variations such as “comprises” or “comprising” will beunderstood to imply the inclusion of stated elements but not theexclusion of any other elements.

Further, in this specification, the phrase “on a plane” means viewing atarget portion from the top, and the phrase “on a cross-section” meansviewing a cross-section formed by vertically cutting a target portionfrom the side.

FIG. 1 is a block diagram of an organic light emitting diode displayaccording to an exemplary embodiment. Referring to FIG. 1 , the organiclight emitting diode display according to an exemplary embodiment mayinclude a gate driver GD, a data driver DD, and a pixel unit 40. Thepixel unit 40 includes a plurality of pixels PX. The pixel PX means aminimum unit for displaying, and the organic light emitting diodedisplay displays an image through the plurality of pixels PX.

The gate driver GD generates a scan signal corresponding to drivingpower and control signals, which are supplied from the outside, andsupplies the scan signal to a gate line 152. The pixels PX are selectedby the scan signal to sequentially receive a data voltage.

The gate driver GD may be a thin film transistor on a substrate alongwith a pixel circuit included in the pixel unit 40 or may be mounted onthe substrate in a chip shape, and may be variously positioned aroundthe pixel unit 40.

The data driver DD generates a data voltage corresponding to data andcontrol signals, which are supplied from the outside, and supplies thedata voltage to a data line 171, and may be variously positioned aroundthe pixel unit 40. The data voltage supplied to the data line 171 issupplied to the pixel PX selected by the scan signal whenever the scansignal is supplied. According to an embodiment, the organic lightemitting diode display may further include a light emission controldriver that supplies a light emission control signal.

The pixel unit 40 includes a plurality of pixel PXs at the intersectionof a gate line 152 and a data line 171. The pixel unit 40 receives adriving voltage ELVDD as a high potential pixel power and a commonvoltage ELVSS as a low potential pixel power from the outside, and thedriving voltage ELVDD and the common voltage ELVSS are transmitted toeach pixel PX.

The pixel PX emits light having a luminance corresponding to a drivingcurrent flowing from the driving voltage ELVDD to the common voltageELVSS corresponding to the data voltage, thereby displaying the image.

Hereinafter, the organic light emitting diode display according to anexemplary embodiment will be described with reference to FIG. 2 and FIG.3 . FIG. 2 is an equivalent circuit diagram of one pixel of an organiclight emitting diode display according to an exemplary embodiment. FIG.3 illustrates a cross-sectional view of an organic light emitting diodedisplay according to an exemplary embodiment

Referring to FIG. 2 , the organic light emitting diode display accordingto an exemplary embodiment includes a plurality of signal lines 152,171, and 172, a plurality of transistors T1 and T2, a storage capacitorCst, and an organic light emitting diode OLED. Also, the organic lightemitting diode display according to the present exemplary embodimentincludes a blocking layer 160 that overlaps the driving transistor T2 ona plane, e.g., along a vertical direction DV orthogonal to the plane, asmay be seen in FIG. 3 .

The signal lines 152, 171, and 172 include a gate line 152 transmittingthe scan signal Sn, a data line 171 transmitting the data voltage Dm,and a driving voltage line 172 transmitting the driving voltage ELVDD.The transistors T1 and T2 include a switching transistor T1 and adriving transistor T2.

The switching transistor T1 includes a gate electrode connected to thegate line 152, a first electrode connected to the data line 171, and asecond electrode connected to a gate electrode of the driving transistorT2. The second electrode of the switching transistor T1 is alsoconnected to a first storage electrode of the storage capacitor Cst. Theswitching transistor T1 is connected to the gate line 152 to be turnedon in response to the scan signal Sn, and when the switching transistorT1 is turned on, the data voltage Dm supplied through the data line 171is supplied to the first storage electrode of the first storagecapacitor Cst.

A first electrode of the driving transistor T2 may be connected to thedriving voltage line 172 and to the blocking layer 160. The gateelectrode of the driving transistor T2 may be connected to the storagecapacitor Cst such that the first storage electrode receiving the firstelectrode driving voltage ELVDD. A second electrode of the drivingtransistor T2 may be connected to the first electrode of the organiclight emitting diode OLED and to a second storage electrode of thestorage capacitor Cst. The driving transistor T2 outputs the drivingcurrent to the organic light emitting diode OLED depending on the datavoltage stored in the storage capacitor Cst.

The storage capacitor Cst includes the first storage electrode and thesecond storage electrode. The first storage electrode of the storagecapacitor Cst is connected to the second electrode of the switchingtransistor T1 and to the gate electrode of the driving transistor T2.The second storage electrode of the storage capacitor Cst is connectedto the second electrode of the driving transistor T2 and to the firstelectrode of the organic light emitting diode OLED. The storagecapacitor Cst may store the data voltage Dm supplied through theswitching transistor T1. The data voltage Dm stored to the storagecapacitor Cst determines a magnitude of the driving current bycontrolling a turn-on degree of the driving transistor T2. While, FIG. 2shows the structure including two transistors and one capacitor, one ormore transistors or capacitors may be additionally included.

The organic light emitting diode OLED includes the first electrodeconnected to the second electrode of the driving transistor T2 and asecond electrode connected to the common voltage ELVSS. The firstelectrode of the organic light emitting diode OLED may be an anode andthe second electrode of the organic light emitting diode OLED may be acathode. The organic light emitting diode OLED emits light depending thedriving current output from the driving transistor T2 and displays agray scale depending on a brightness degree.

The blocking layer 160 is electrically connected to the driving voltageline 172 that transmits the driving voltage ELVDD. The blocking layer160 may be formed of a metal having a conductive characteristic or asemiconductor material having a conductive characteristic equivalentthereto. The driving voltage ELVDD is constantly applied to the blockinglayer 160 to prevent the potential from being changed while a certaincharge is injected into the blocking layer 160. The blocking layer 160has a function like the second gate electrode of the driving transistorT2 in a circuit diagram. However, since the predetermined voltage isapplied to the blocking layer 160, the driving transistor T2 is notturned on, but the driving transistor T2 has a predeterminedcharacteristic.

Referring to FIG. 3 , the organic light emitting diode display accordingto the present exemplary embodiment includes a substrate 110, theswitching transistor T1, the driving transistor T2, and the organiclight emitting diode OLED.

The substrate 110 may include a flexible material that may be bent,curved, and folded such as plastic. For example, the substrate 110 maybe made of a polymer, e.g., polyimide, polyamide, polycarbonate,polyethylene terephthalate, or the like. The substrate 110 may be arigid substrate made of a material such as glass.

A first buffer layer 121 is on the substrate 110. The first buffer layer121 may include a silicon nitride (SiN_(x)), a silicon oxide (SiO_(x)),etc. As the first buffer layer 121 is between the substrate 110 and afirst semiconductor layer 136 a, an impurity is prevented from beingdiffused into the first semiconductor layer 136 a from the substrate 110and planarizes the substrate 110, thereby alleviating stress on thefirst semiconductor layer 136 a formed on the first buffer layer 121.

The first semiconductor layer 136 a is on the first buffer layer 121.The first semiconductor layer 136 a may be made of a polysilicon. Forexample, the first semiconductor layer 136 a may be made of apolysilicon formed by crystallizing amorphous silicon by acrystallization method such as excimer laser annealing (ELA). The firstsemiconductor layer 136 a may include a first channel region 134 a, afirst source region 133 a, and a first drain region 135 a. The firstsource region 133 a and the first drain region 135 a are on respectivesides of the first channel region 134 a. The first channel region 134 ais an intrinsic semiconductor in which an impurity is not doped, and thefirst source region 133 a and the first drain region 135 are impuritysemiconductors in which a conductive impurity is doped. The conductiveimpurity may be a P-type impurity.

A first gate insulating layer 122 is on the first semiconductor layer136 a. The first gate insulating layer 122 may be a single layerincluding at least one of a silicon nitride (SiN_(x)), a silicon oxide(SiO_(x)), a multilayer thereof, or the like.

A first gate electrode 155 a and the blocking layer 160 are on, e.g.,directly on, the first gate insulating layer 122. The blocking layer 160is in the same layer as the first gate electrode 155 a, e.g., directlyon the first gate insulating layer 122, but may be closer to thesubstrate 110 along the vertical direction DV due to the firstsemiconductor layer 136 a being between the first gate electrode 155 aand the substrate 110.

The first gate electrode 155 a may be a multilayer including a metallayer which includes at least one of copper (Cu), a copper alloy,aluminum (Al), an aluminum alloy, molybdenum (Mo), and a molybdenumalloy. The first gate electrode 155 a and the first semiconductor layer136 a may configure the switching transistor T1.

The blocking layer 160 is formed together in the same layer as the firstgate electrode 155 a, e.g., without an additional mask process, therebysimplifying the process. The blocking layer 160 is electricallyconnected to the driving voltage line 172 through an opening to receivethe driving voltage ELVDD. The blocking layer 160 may be formed of ametal having a conductive characteristic, or a semiconductor materialhaving a conductive characteristic equivalent thereto.

A second buffer layer 141 is on the first gate electrode 155 a, theblocking layer 160, and the first gate insulating layer 122. The secondbuffer layer 141 may include a silicon nitride (SiN_(x)), a siliconoxide (SiO_(x)), etc. The second buffer layer 141 has openingsrespectively exposing the first source region 133 a and the first drainregion 135 a, as well as the blocking layer 160.

The second semiconductor layer 136 b is on the second buffer layer 141.The second semiconductor layer 136 b includes a second channel region134 b, a second source region 133 b, and a second drain region 135 b.The second source region 133 b and the second drain region 135 b are onrespective sides of the second channel region 134 b.

The second semiconductor layer 136 b may be made of an oxidesemiconductor. Although the first semiconductor layer 136 a is made ofthe polysilicon and the second semiconductor layer 136 b is the oxidesemiconductor, the first semiconductor layer 136 a of the switchingtransistor T1 may be formed of the oxide semiconductor having an offcurrent characteristic that is better than that of the polysilicon, andthe second semiconductor layer 136 b of the driving transistor T2 may beformed of the polysilicon. Alternatively, both the first semiconductorlayer 136 a and the second semiconductor layer 136 b may be made of thesame material, e.g., polysilicon or oxide semiconductor.

A second gate insulating layer 123 is on the second semiconductor layer136 b. The second gate insulating layer 123 may be a single layerincluding at least one of a silicon nitride (SiN_(x)), a silicon oxide(SiO_(x)), the multilayer, or the like.

A first source electrode 161 a and a first drain electrode 162 a are onthe second gate insulating layer 123. The first source electrode 161 aand the first drain electrode 162 a are respectively connected to thefirst source region 133 a and the first drain region 135 a of the firstsemiconductor layer 136 a through the openings formed in the first gateinsulating layer 122, the second buffer layer 141, and the second gateinsulating layer 123.

A second gate electrode 155 b overlapping the second channel region 134b is also on the second gate insulating layer 123. The second gateelectrode 155 b may be a multilayer including a metal layer whichincludes at least one of copper (Cu), a copper alloy, aluminum (Al), analuminum alloy, molybdenum (Mo), a molybdenum alloy, or the like. Thesecond gate electrode 155 b and the second semiconductor layer 136 b mayconfigure the driving transistor T2.

In the organic light emitting diode display according to the presentexemplary embodiment, the switching transistor T1 and the drivingtransistor T2 are on the different layers. In other words, the firstsemiconductor layer 136 a of the switching transistor T1 and the secondsemiconductor layer 136 b of the driving transistor T2 are on thedifferent layers. Therefore, an interval along a horizontal direction DHbetween the switching transistor T1 and the driving transistor T2 may benarrowed or these transistors may partially overlap along the verticaldirection DV, thereby increasing a degree of design freedom, increasingan aperture ratio of the pixel, and/or increasing the resolution of thedisplay device.

The blocking layer 160 may overlap the second channel region 134 b ofthe driving transistor T2 along the vertical direction DV. The blockinglayer 160 may also overlap the second source region 133 b and the seconddrain region 135 b at respective sides of the second channel region 134b, e.g., may overlap an entirety of the second semiconductor layer 136b. Also, the blocking layer 160 may not overlap the first channel region134 a of the switching transistor T1. As the blocking layer 160 is underthe driving transistor T2, a kickback voltage due to a parasiticcapacitance of the switching transistor T1 and the driving transistor T2may be minimized, and the display quality of the organic light emittingdiode display may be improved by preventing a residual image, asdescribed in detail below. In addition, the blocking layer 160 below thesecond semiconductor layer 136 b is not floated, but receives thedriving voltage ELVDD, e.g., may extend beyond the second semiconductorlayer 136 b along the horizontal direction DH to receive the drivingvoltage ELVDD, thereby preventing deterioration of the display qualitydue to the unnecessary parasitic capacitance.

A third gate insulating layer 124 is on the second gate insulating layer123, the first source electrode 161 a, the first drain electrode 162 a,and the second gate electrode 155 b. A second drain electrode 162 b anda second storage electrode 165 that extends from the second drainelectrode 162 b along the horizontal direction DH are on the third gateinsulating layer 124.

The second drain electrode 162 b is connected to the second drain region135 b of the driving transistor T2 through the opening formed in thesecond gate insulating layer 123 and the third gate insulating layer124. The second storage electrode 165 extends from the second drainelectrode 162 b along the horizontal direction DH to overlap the secondgate electrode 155 b. The second gate electrode 155 b and the secondstorage electrode 165 overlap each other along the vertical direction DVvia the third gate insulating layer 124 to form the storage capacitorCst. In this case, the second gate electrode 155 b may be the firststorage electrode of the storage capacitor Cst. The third gateinsulating layer 124 is a dielectric material, and the storagecapacitance is determined by the charge charged in the storage capacitorCst and the voltage between the first storage electrode 155 b and thesecond storage electrode 165.

An interlayer insulating layer 142 is on the third gate insulating layer124 and the second storage electrode 165. The interlayer insulatinglayer 142 may be the single layer including at least one of a siliconnitride (SiN_(x)) a silicon oxide (SiO_(x)), the multilayer, or thelike.

A second source electrode 161 b and a connecting member 174 are on theinterlayer insulating layer 142. The second source electrode 161 b isconnected to the second source region 133 b of the second semiconductorlayer 136 b through the opening in the interlayer insulating layer 142,the second gate insulating layer 123, and the third gate insulatinglayer 124. The connecting member 174 is connected to the second drainelectrode 162 b of the driving transistor T2 and the second storageelectrode 165 of the storage capacitor Cst through the opening in theinterlayer insulating layer 142.

The second source electrode 161 b extends and is connected to thedriving voltage line 172 to receive the driving voltage ELVDD. Thesecond source electrode 161 b is also connected to the blocking layer160 through the opening in the second buffer layer 141, the second gateinsulating layer 123, the third gate insulating layer 124, and theinterlayer insulating layer 142. Accordingly, the blocking layer 160 isnot floated and receives the driving voltage ELVDD.

The data line 171 is also on the interlayer insulating layer 142. Thedata line 171 is connected to the first source electrode 161 a of theswitching transistor T1 through the opening in the third gate insulatinglayer 124 and the interlayer insulating layer 142 to transmit the datavoltage to the switching transistor T1.

A passivation layer 180 is on the interlayer insulating layer 142, thedata line 171, the second source electrode 161 b, and the connectingmember 174. The passivation layer 180 covers the interlayer insulatinglayer 142, the second source electrode 161 b, the data line 171, and theconnecting member 174 to provide a flat upper surface so that a pixelelectrode 191 may be formed without a step on the passivation layer 180.The passivation layer 180 may be formed of an organic material, e.g., apolyacrylate resin, a polyimide resin, or the like, or a laminated filmof an organic material and an inorganic material.

The pixel electrode 191 is on the passivation layer 180. The pixelelectrode 191 is connected to the connecting member 174 through theopening formed in the passivation layer 180. Accordingly, the pixelelectrode 191 is connected to the second drain electrode 162 b of thedriving transistor T2 through the connecting member 174. The drivingtransistor T2 is connected to the pixel electrode 191 to supply thedriving current to the organic light emitting element.

A partition wall 361 covers the passivation layer 180 and the pixelelectrode 191, and has a pixel opening 365 exposing the pixel electrode191. The partition wall 361 may include an organic material, e.g., apolyacrylate resin, a polyimide resin, or the like, or an inorganicmaterial, e.g., a silica-based inorganic material.

An organic emission layer 370 is on the pixel electrode 191 exposed bythe pixel opening 365. The organic emission layer 370 may be formed of alow molecular organic material and a high molecular organic materialsuch as or PEDOT (poly(3,4-ethylenedioxythiophene)), etc. Also, theorganic emission layer 370 may be a multilayer further including atleast one among a hole injection layer (HIL), a hole transporting layer(HTL), an electron transporting layer (ETL), an electron injection layer(EIL), or the like. The organic emission layer 370 may include a redorganic emission layer emitting red light, a green organic emissionlayer emitting green light, and a blue organic emission layer emittingblue light.

A common electrode 270 is on the organic emission layer 370. The commonelectrode 270 may cover a plurality of pixels. The pixel electrode 191,the organic emission layer 370, and the common electrode 270 may formthe organic light emitting diode OLED.

Here, the pixel electrode 191 may be the anode of the hole injectionelectrode, and the common electrode 270 may be the cathode of theelectron injection electrode. Alternatively, depending on a drivingmethod of the organic light emitting diode display, the pixel electrode191 may be the cathode and the common electrode 270 may be the anode.Holes and electrons are respectively injected from the pixel electrode191 and the common electrode 270 into the organic emission layer 370,and light is emitted when excitons which are combinations of theinjected holes and electrons fall from an excited state to a groundstate.

An encapsulation layer 390 is on the common electrode 270. The thin filmencapsulation layer 390 encapsulates the organic light emitting diodeOLED, thereby preventing penetration of external moisture and oxygen, asthe organic light emitting diode OLED is very vulnerable to the moistureand oxygen. The encapsulation layer 390 may include a plurality oflayers, and may be formed of a composite film including both aninorganic film and an organic film. The encapsulation layer 390 may beformed of a triple layer in which an inorganic film, an organic film,and an inorganic film are sequentially formed.

FIG. 4 is a graph showing an instantaneous afterimage duration of anorganic light emitting diode display according to a comparative exampleand an exemplary embodiment. In FIG. 4 , A represents the organic lightemitting diode display according to the comparative example without ablocking layer, and B represents the organic light emitting diodedisplay including a blocking layer that overlaps the channel region ofthe driving transistor according to an exemplary embodiment.

In the organic light emitting diode display, if black and white aredisplayed for 10 seconds and then a low gray is simultaneously driven, adifference in luminance occurs in the region displaying black and theregion displaying white even though the same colors are driven. Indetail, the region displaying black is brighter than the gray to bedriven immediately after the gray driving of the low gray, and theregion displaying white is darker than the gray to be driven. Theinstantaneous afterimage means an afterimage recognized by the luminancedifference.

The duration of the instantaneous afterimage of FIG. 4 is found bymeasuring the time that the difference in luminance between the regiondisplaying black from immediately after the gray driving of the low grayand the region displaying white reaches 0.4%. Here, the luminancedifference is calculated by (B−W)/(B+W), where B is the luminance of theregion displaying black and W is the luminance of the region displayingwhite. The instantaneous afterimage is directly related to the displayquality of the display device, so that if the duration of theinstantaneous afterimage is reduced, a clearer image may be achieved.This instantaneous afterimage may be improved by minimizing verticalcross-talk between the transistors in the organic light emitting diodedisplay.

Referring to FIG. 4 , in the case A, the instantaneous afterimage lastsfor 7.66151 seconds. However, in the case B, the instantaneousafterimage lasts for only 4.42379 seconds. Accordingly, an organic lightemitting diode display including the blocking layer according toembodiments improves the instantaneous afterimage by about 3.2 secondscompared with the organic light emitting diode display (the comparativeexample) without the blocking layer.

Next, the organic light emitting diode display according to an exemplaryembodiment will be described with reference to FIG. 5 and FIG. 6 . FIG.5 is an equivalent circuit diagram of one pixel of an organic lightemitting diode display according to another exemplary embodiment.

Referring to FIG. 5 , the organic light emitting diode display accordingto an exemplary embodiment includes signal lines 152, 171, and 172, twotransistors T1 and T2, the storage capacitor Cst, and the organic lightemitting diode OLED. Also, the organic light emitting diode displayaccording to the present exemplary embodiment includes the blockinglayer 160 that overlaps the driving transistor T2 on a plane, e.g.,along the vertical direction DV. Except for the second storage electrodeof the storage capacitor Cst, which is connected to the driving voltageline 172 and the first electrode of the driving transistor T2, thisembodiment is similar to that of FIG. 2 , and redundant description isnot repeated.

The storage capacitor Cst includes the first storage electrode and thesecond storage electrode. The first storage electrode of the storagecapacitor Cst is connected to the second electrode of the switchingtransistor T1 and the gate electrode of the driving transistor T2. Thesecond storage electrode of the storage capacitor Cst is connected tothe driving voltage line 172. The second storage electrode of thestorage capacitor Cst is also connected to the first electrode of thedriving transistor T2 and the blocking layer 160.

The storage capacitor Cst may store the data voltage Dm supplied throughthe switching transistor T1. The data voltage Dm stored in the storagecapacitor Cst determines the magnitude of the driving current byadjusting the degree to which the driving transistor T2 turns on.

The blocking layer 160 is electrically connected to the driving voltageline 172 that transmits the driving voltage ELVDD. The driving voltageELVDD is constantly applied to the blocking layer 160 to prevent thepotential from being changed while a certain charge is injected into theblocking layer 160. The blocking layer 160 has a function like thesecond gate electrode of the driving transistor T2 in a circuit diagram.However, since the predetermined voltage is applied to the blockinglayer 160, the driving transistor T2 is not turned on, but the drivingtransistor T2 has a predetermined characteristic.

FIG. 6 is a cross-sectional view of an organic light emitting diodedisplay according to another exemplary embodiment. The configuration ofFIG. 6 is similar to that of FIG. 3 such that the detailed descriptionoverlapping with FIG. 3 is not repeated.

Referring to FIG. 6 , the organic light emitting diode display accordingto the present exemplary embodiment includes the substrate 110, theswitching transistor T1, the driving transistor T2, and the organiclight emitting diode OLED.

The second source electrode 161 b extends and is connected to thedriving voltage line 172 to receive the driving voltage ELVDD. Thesecond source electrode 161 b is also connected to the blocking layer160 through the opening in the second buffer layer 141, the second gateinsulating layer 123, the third gate insulating layer 124, and theinterlayer insulating layer 142. Accordingly, the blocking layer 160 isnot floated and receives the driving voltage ELVDD.

Additionally, different from FIG. 3 , the second source electrode 161 bis also connected to the second storage electrode 165 of the storagecapacitor Cst through the opening formed in the interlayer insulatinglayer 142, and the second storage electrode 165 is not connected to thesecond drain electrode 162 b. Accordingly, the second storage electrode165 of the storage capacitor Cst receives the driving voltage ELVDD.Otherwise, the remainder of the structure is the same as in FIG. 3 .

FIG. 7 is a cross-sectional view of an organic light emitting diodedisplay according to another exemplary embodiment. Referring to FIG. 7 ,the organic light emitting diode display according to the presentexemplary embodiment includes the first transistor T1, the secondtransistor T2, and a blocking layer 260.

The organic light emitting diode display according to the presentexemplary embodiment includes a substrate 210 and a first buffer layer221 on the substrate 210.

A first semiconductor layer 236 a is on the first buffer layer 221. Thefirst semiconductor layer 236 a may be made of polysilicon, and includesa first channel region 234 a, a first source region 233 a, and a firstdrain region 235 a. The first source region 233 a and the first drainregion 235 a are at respective sides of the first channel region 234 a.

A first gate insulating layer 222 is on the first semiconductor layer236 a and a first gate electrode 255 a is on the first gate insulatinglayer 222. The first gate electrode 255 a overlaps the first channelregion 234 a of the first semiconductor layer 236 a. The first gateelectrode 255 a and the first semiconductor layer 236 a may form thefirst transistor T1.

A second gate insulating layer 223 is on the first gate electrode 255 a.A second gate electrode 265 and the blocking layer 260 are on the secondgate insulating layer 223. The second gate electrode 265 overlaps thefirst gate electrode 255 a and the first channel region 234 a of thefirst semiconductor layer 236 a along the vertical direction DV. Alongthe vertical direction DV, the first gate electrode 255 a and the secondgate electrode 265 overlap and are spaced apart by the second gateinsulating layer 223, thereby forming the storage capacitor.

The blocking layer 260 is in the same layer, e.g., directly on thesecond gate insulating layer 223, as the second gate electrode 265, butmay be closer to the substrate 210 along the vertical direction DV dueto the first semiconductor layer 236 a being between the second gateelectrode 265 and the substrate 210. The blocking layer 260 iselectrically connected to a driving voltage line 272 through the openingto receive the driving voltage ELVDD. The blocking layer 260 may beformed of the metal having the conductive characteristic or thesemiconductor material having the conductive characteristic equivalentthereto.

A second buffer layer 241 is on the second gate electrode 265, theblocking layer 260, and the second gate insulating layer 223. A secondsemiconductor layer 236 b is on the second buffer layer 241. The secondsemiconductor layer 236 b includes a second channel region 234 b, asecond source region 233 b, and a second drain region 235 b. The secondsource region 233 b and the second drain region 235 b are at respectivesides of the second channel region 234 b. The second semiconductor layer236 b may be made of the oxide semiconductor.

A third gate insulating layer 224 is on the second semiconductor layer236 b. A third gate electrode 255 b overlapping the second channelregion 234 b is on the third gate insulating layer 224. The third gateelectrode 255 b and the second semiconductor layer 236 b may form thesecond transistor T2.

The blocking layer 260 overlaps the second channel region 234 b of thesecond transistor T2. The blocking layer 260 may also overlap the secondsource region 233 b and the second drain region 235 b at respectivesides of the second channel region 234 b, e.g., may overlap an entiretyof the second semiconductor layer 236 b. Also, the blocking layer 260may not overlap the first channel region 234 a of the first transistorT1.

It is described that the first semiconductor layer 236 a of the firsttransistor T1 is made of the polysilicon and the second semiconductorlayer 236 b of the second transistor T2 is made of the oxidesemiconductor, but it is possible for the first semiconductor layer 236a to be made of the oxide semiconductor and the second semiconductorlayer 236 b to be made of the polysilicon. Alternatively, both the firstsemiconductor layer 236 a and the second semiconductor layer 236 b maybe made of the same material, e.g., polysilicon or oxide semiconductor.

An interlayer insulating layer 242 is on the third gate electrode 255 band the third gate insulating layer 224. A first source electrode 261 a,a first drain electrode 262 a, a second source electrode 261 b, a seconddrain electrode 262 b, and the driving voltage line 272 are on theinterlayer insulating layer 242.

The first source electrode 261 a and the first drain electrode 262 a arerespectively connected to the first source region 233 a and the firstdrain region 235 a of the first semiconductor layer 236 a through theopenings in the second buffer layer 241, first to third gate insulatinglayers 222 to 224, and the interlayer insulating layer 242.

Further, the second source electrode 261 b and the second drainelectrode 262 b are respectively connected to the second source region233 b and the second drain region 235 b of the second semiconductorlayer 236 b through the openings in the interlayer insulating layer 242and the third gate insulating layer 224.

Finally, the driving voltage line 272 is connected to the blocking layer260 through the opening in the second buffer layer 241, the interlayerinsulating layer 242, and the third gate insulating layer 224.

By way of summation and review, embodiments herein may provide anorganic light emitting diode display having increased design freedom,high resolution, and improved display quality.

Example embodiments have been disclosed herein, and although specificterms are employed, they are used and are to be interpreted in a genericand descriptive sense only and not for purpose of limitation. In someinstances, as would be apparent to one of ordinary skill in the art asof the filing of the present application, features, characteristics,and/or elements described in connection with a particular embodiment maybe used singly or in combination with features, characteristics, and/orelements described in connection with other embodiments unless otherwisespecifically indicated. Accordingly, it will be understood by those ofskill in the art that various changes in form and details may be madewithout departing from the spirit and scope of the present invention asset forth in the following claims.

What is claimed is:
 1. An organic light emitting diode display,comprising: a substrate; a first buffer layer on the substrate; a firstsemiconductor layer on the first buffer layer; a first gate insulatinglayer on the first semiconductor layer; a first gate electrode on thefirst gate insulating layer; a second gate insulating layer on the firstgate electrode; a second gate electrode and a blocking layer on thesecond gate insulating layer; a second buffer layer on the second gateelectrode and the blocking layer; a second semiconductor layer on thesecond buffer layer, the second semiconductor layer including a channelregion; a third gate insulating layer on the second semiconductor layer;and a third gate electrode on the third gate insulating layer, whereinthe blocking layer is disposed in a same layer as the second gateelectrode, the blocking layer overlapping the channel region of thesecond semiconductor layer in a direction perpendicular to a surface ofthe substrate.
 2. The organic light emitting diode display as claimed inclaim 1, further comprising: an interlayer insulating layer on the thirdgate electrode; a first electrode and a second electrode on theinterlayer insulating layer, wherein the first electrode is in contactwith the first semiconductor layer, and the second electrode is incontact with the second semiconductor layer.
 3. The organic lightemitting diode display as claimed in claim 2, wherein the firstelectrode and the second electrode are in contact with a top surface ofthe interlayer insulating layer.
 4. The organic light emitting diodedisplay as claimed in claim 3, further comprising: a driving voltageline transmitting a driving voltage, wherein the blocking layer iselectrically connected to the driving voltage line.
 5. The organic lightemitting diode display as claimed in claim 2, wherein the second gateelectrode overlaps the first gate electrode in a direction perpendicularto the surface of the substrate.
 6. The organic light emitting diodedisplay as claimed in claim 2, further comprising: a passivation layeron the first electrode and the second electrode; a pixel electrode onthe passivation layer; an organic emission layer on the pixel electrode;and a common electrode on the organic emission layer.
 7. The organiclight emitting diode display as claimed in claim 6, wherein the pixelelectrode is electrically connected to the second electrode.
 8. Theorganic light emitting diode display as claimed in claim 7, furthercomprising: an encapsulation layer on the common electrode.
 9. Theorganic light emitting diode display as claimed in claim 1, wherein oneof the first semiconductor layer and the second semiconductor layer isan oxide semiconductor and the other includes polysilicon.
 10. Theorganic light emitting diode display as claimed in claim 1, wherein theblocking layer does not overlap the second gate electrode in a directionperpendicular to the surface of the substrate.
 11. The organic lightemitting diode display of claim 1, further comprising a driving voltageline transmitting a driving voltage, wherein the blocking layer iselectrically connected to the driving voltage line.
 12. The organiclight emitting diode display of claim 11, further comprising aninterlayer insulating layer disposed on the third gate electrode,wherein the driving voltage line is disposed on the interlayerinsulating layer.
 13. The organic light emitting display device of claim1, wherein the blocking layer is comprised of a metal having aconductive characteristic.
 14. The organic light emitting display deviceof claim 1, wherein the blocking layer to minimize a kickback voltagedue to parasitic capacitance of a first transistor and a secondtransistor, wherein the first transistor comprises the firstsemiconductor layer and the first gate electrode and the secondtransistor comprises the second semiconductor layer and the third gateelectrode.
 15. The organic light emitting display device of claim 1,wherein the blocking layer is connected to a driving voltage line toreceive a driving voltage to prevent deterioration of display qualitydue to unnecessary parasitic capacitance of a first transistor thatcomprises the first semiconductor layer and of a second transistor thatcomprises the second semiconductor layer.
 16. An organic light emittingdiode display, comprising: a substrate; a first buffer layer on thesubstrate; a first semiconductor layer on the first buffer layer; afirst gate insulating layer on the first semiconductor layer; a firstgate electrode on the first gate insulating layer; a second gateinsulating layer on the first gate electrode; a second gate electrodeand a blocking layer on the second gate insulating layer; a secondbuffer layer on the second gate electrode and the blocking layer; asecond semiconductor layer on the second buffer layer, the secondsemiconductor layer including a channel region; a third gate insulatinglayer on the second semiconductor layer; and a third gate electrode onthe third gate insulating layer, wherein the blocking layer is disposedin a same layer as the second gate electrode, the blocking layeroverlapping the channel region of the second semiconductor layer in adirection perpendicular to a surface of the substrate, wherein theblocking layer comprises a same material as the second gate electrode.17. An organic light emitting diode display, comprising: a substrate; afirst buffer layer on the substrate; a first semiconductor layer on thefirst buffer layer; a first gate insulating layer on the firstsemiconductor layer; a first gate electrode on the first gate insulatinglayer; a second gate insulating layer on the first gate electrode; asecond gate electrode and a blocking layer on the second gate insulatinglayer; a second buffer layer on the second gate electrode and theblocking layer; a second semiconductor layer on the second buffer layer,the second semiconductor layer including a channel region; a third gateinsulating layer on the second semiconductor layer; and a third gateelectrode on the third gate insulating layer, wherein the blocking layeris disposed in a same layer as the second gate electrode, the blockinglayer overlapping the channel region of the second semiconductor layerin a direction perpendicular to a surface of the substrate, wherein theblocking layer is electrically conductive.